Theoretical study of two-dimensional tetragonal transition metal chalcogenides and the potassium derivatives

In this work, we report a theoretical study on a series of two-dimensional (2D) tetragonal transition metal chalcogenide monolayers. By using first-principles calculations, twenty-two MX (M = transition metals, X = S, Se, Te) monolayers are found to be dynamically stable. Among the stable monolayers, five of them are semiconductors, namely ZnS, ZnSe, ZnTe, CdS, and CdSe, and the remaining have a metallic behavior. The optical absorbance demonstrates that the photoabsorption peaks of CdX monolayers are red-shifted compared with those of their ZnX counterparts, indicating that CdX monolayers are more efficient in sunlight harvesting. We also studied a series of tetragonal K_nMX (n = 0.5 or 1) monolayers, and found that KAgS, KAgSe, and KAgTe become semiconductors with moderate band gaps. Furthermore, the electronic transport properties were investigated through the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT) for the proposed MX-based devices. It is anticipated that these novel tetragonal transition metal chalcogenide monolayers would be excellent candidates for abundant potential applications in areas of nanoscale devices, and energy storage and conversion.